Inaccuracy in measuring solid armature velocity by B-dot probe in electromagnetic launch

The B-dot probe is one of the most common measurements of velocity for armatures in the bore of a railgun. In this paper, we analyzed the principle of B-dot probe induced voltage, and found that the rate of change of armature current is the inaccuracy factor in locating moving armature by B-dot probe. Armature currents with different dI/dt were set in the Matlab simulation of locating moving armature by B-dot probe. The results show that the error in locating moving armature by B-dot probe is positively correlated to the absolute value of dI/dt, and is negatively correlated to the product of armature current and armature velocity if dI/dts are equal. According to the results, we performed a further simulation of inaccuracy in B-dot-probe measurement of armature velocity, and its results could be divided into three types. Real current data were utilized to replace the ideal current in the simulation, the results of which well matched the ideal one. In short, the error in measuring armature velocity by B-dot probe is small if dI/dts are similar, and the error is large if dI/dts are different.